Ultrasound is a diagnostic imaging technique. In using the ultrasound technique, sound is transmitted into a patient from a transducer. When the sound waves propagate through the body, they interact with tissues and fluids in the body and are partially or wholly reflected, or absorbed, depending on the acoustic properties of the tissues and fluids. Sound waves reflected from within the body can be detected by the receiver in the transducer and processed to form an image, the contrast of which depends on the different acoustic properties of the tissues and fluids within the body.
The principles underlying image formation in ultrasound have directed researchers to develop gaseous contrast agents. Changes in acoustic properties or acoustic impedance are most pronounced at interfaces of different substances with greatly differing density or acoustic impedance, particularly at the interface between solids, liquids and gases. When sound waves encounter such interfaces, the changes in acoustic impedance result in a more intense reflection of sound waves and a more intense signal in the ultrasound image. An additional factor affecting the efficiency or reflection of sound is the elasticity of the reflecting interface, with more elastic surfaces resulting in more efficient reflection of sound. Gas bubbles present highly elastic gas-liquid interfaces. To improve the quality of ultrasound images, researchers have been able to develop ultrasound contrast agents based on gas bubbles. The use of ultrasound contrast agents is discussed in the treatise Ultrasound Contrast Agents: Basic Principles and Clinical Applications by B. B. Goldberg, et al. (Eds.), Taylor & Francis (2nd Edition, 2001).
Another area of significant research effort is in the area of targeted drug delivery. One of the major challenges is achieving the systemic delivery of nucleic acids directly into a tissue, for example, for gene therapy. This requires developing a vehicle that is able to protect the nucleic acid from degradation, while delivering the gene of interest to the specific tissue and specific subcellular compartment.
Viruses are attractive delivery vectors for genetic material because of their ability to efficiently transfer genes with sustained expression. Recombinant adenoviruses are one of the most common gene transfer vectors utilized in human clinical trials, but systemic administration of this virus will also be met by host innate and adaptive antiviral immune responses which can limit and/or preclude repetitive regimens. See H. Jiang, et al. “Recombinant adenovirus vectors activate the alternative complement pathway, leading to the binding of human complement protein C3 independent of anti-ad antibodies”, Mol. Ther., 2004, 10(6), 1140-42.
The use of ultrasound contrast agents has been suggested as a means of delivering genetic material to tissues. See R. Bekeredjian, et al., “Ultrasound-targeted microbubble destruction can repeatedly direct highly specific plasmid expression to the heart”, Circulation, 2003, 108(8): 1022-26; R. Bekeredjian, et al., “Use of ultrasound contrast agents for gene or drug delivery in cardiovascular medicine”, J. Am. Coll Cardiol., 2005, 45(3), 329-35; P. A. Dijkmans, et al., “Microbubbles and ultrasound: from diagnosis to therapy”, Eur. J. Echocardiogr., 2004, 5(4), 245-56; H. Hosseinkhani, et al., “Ultrasound enhances the transfection of plasmid DNA by non-viral vectors”, Curr. Pharm. Biotechnol., 2003, 4(2), 109-22; I. V. Larina, et al., “Enhancement of drug delivery in tumors by using interaction of nanoparticles with ultrasound radiation”, Technol. Cancer Res. Treat., 2005, 4(2), 217-26; I. Lavon, et al., “Ultrasound and transdermal drug delivery”, Drug Discov. Today, 2004, 9(15), 670-76; A. Lawrie, et al., “Microbubble-enhanced ultrasound for vascular gene delivery”, Gene Ther., 2000, 7(23), 2023-27; A. Lawrie, et al., “Ultrasound enhances reporter gene expression after transfection of vascular cells in vitro”, Circulation, 1999, 99(20), 2617-20; D. L. Miller, et al., “Ultrasonic enhancement of gene transfection in murine melanoma tumors”, Ultrasound Med. Biol., 1999, 25(9), 1425-30; C. M. Newman, et al., “Ultrasound gene therapy: on the road from concept to reality”, Echocardiography, 2001, 18(4), 339-47; K. Y. Ng, et al., “Therapeutic ultrasound: its application in drug delivery”, Med. Res. Rev., 2002, 22(2), 204-23; M. Shimamura, et al., “Development of efficient plasmid DNA transfer into adult rat central nervous system using microbubble-enhanced ultrasound”, Gene Ther., 2004, 11(20), 1532-39, E. C. Unger, et al., “Gene delivery using ultrasound contrast agents”, Echocardiography, 2001, 18(4), 355-61; E. C. Unger, et al., “Ultrasound enhances gene expression of liposomal transfection”, Invest. Radiol., 1997, 32(12), 723-27; S. D. Voss, et al., “Gene therapy: a primer for radiologists”, Radiographics, 1998, 18(6), 1343-72; W. Wei, et al., “A novel approach to quantitative ultrasonic naked gene delivery and its non-invasive assessment”, Ultrasonics, 2004, 43(2), 69-77.
The theory of such approaches is that genetic material can be loaded into the contrast agent bubbles and then released and incorporated into cells when the bubbles of the contrast agent are ruptured by exposure to ultrasound. Unfortunately, where delivery of viruses encapsulated in microbubbles has been attempted, the delivery has been observed as being non-selective. For example, in experiments where ultrasound-guided delivery of recombinant adenovirus containing β-galactosidase was achieved to the heart in rats, the livers of all animals that received the virus also showed extensive β-galactosidase activity. R. V. Shohet, et al., “Echocardiographic Destruction of Albumin Microbubbles Directs Gene Delivery to the Myocardium”, Circulation, 2000, 101, 2254-56. Therefore, improved methods are needed to achieve site-selective delivery of viruses within the body of an animal.